1. Field of the Invention
The present invention relates to power conversion, and more specifically, to a synchronous rectifier circuit.
2. Description of the Related Art
Power converters are used in a wide variety of electrical equipment. A rectification circuit is a common component of a power converter that converts alternating current (AC) to direct current (DC). Recently, many power supply designers have adopted synchronous rectification systems, characterized in that they use MOSFETs (metal-oxide-semiconductor field-effect transistors) to achieve the rectification function conventionally performed by diodes. Synchronous rectifier circuits provide improved power efficiency and reliability over conventional rectifier circuits, and can also decrease overall system cost.
FIG. 1 is a circuit diagram illustrating a conventional synchronous rectifier circuit 100. The synchronous rectifier circuit 100 comprises input capacitor C1, transformer 118, switches Q1 and Q2 (e.g., N-type MOSFET switches), and gate drivers 114, 116. Transformer 118 includes magnetic core 120, primary winding N1, and secondary windings N2, N3. Input capacitor C1 operates as an input filter to block the DC component of input voltage Vin and pass the AC component of voltage VinAC to transformer 118. The AC input voltage VinAC across primary winding N1 of transformer 118 induces a voltage across secondary windings N2, N3. When VinAC is positive, Vctrl1 turns on switch Q1 via gate driver 114 and Vctrl2 turns off switch Q2 via gate driver 116. Current i1 is induced in secondary winding N2 and flows from the start of secondary winding N2 to the output terminal 119 producing a positive output voltage Vout. When VinAC becomes negative, Vctrl1 turns off switch Q1 via gate driver 114 and Vctrl2 turns on switch Q2 via gate driver 116. Current i2 flows from the end of secondary winding N3 to the output terminal 118 producing a positive output voltage Vout. Thus, output voltage Vout always has a positive polarity even though the input voltage Vin alternates between positive and negative polarities.
A problem with the conventional synchronous rectifier circuit 100 is that its efficiency decreases as the frequency of AC input signal VinAC rises due to skin effects and proximity effects. Skin effects cause the AC current to distribute itself near the outer layer of the conducting components. This increases the effective resistance of the conductors, which in turn, increases power loss of the conventional synchronous rectifier circuit 100. Proximity effects further increase the effective resistance of conductors within closely wound coils (such as windings N1, N2, N3) by constraining the currents to smaller regions of the conductor. Both skin effects and proximity effects increase with frequency, and at high frequencies, the power loss can become substantial.